Origin of reflections from the Brevard Fault Zone

Abstract

The Brevard fault zone is a belt of mylonitic metamorphic rocks that extends northeast over 700 km from Alabama into Virginia. Seismic reflection studies show that the fault zone produces excellent reflections which dip gently to the southeast and sole at depth. The origin of these reflections has been investigated by measuring compressional wave velocities to 800 MPa confining pressure in 127 samples obtained from drill core which continuously sampled the fault zone to a depth of over 300 m. Densities are reported as well as chemical analyses at selected depths. Anisotropy at elevated pressures ranges within the fault zone from less than 5% to over 26%. Similarly, the fault zone rocks show significant chemical variability, with SiO2 ranging from 58.0 to 70.4% by weight. Synthetic seismograms generated from the velocity and density measurements show that the reflections originate within the Brevard fault zone from the complex interaction of compositional variations and seismic anisotropy resulting from ductile strain. Essential to this study has been the continuous coring of a major section of the Brevard fault zone, which has provided the necessary stratigraphic control as well as unweathered samples for laboratory studies.